For the first time, a reference Appl. Phys. Lett. Vol. 59 (1991), p.1272, reported pulse oscillation of 490 nm (blue-green) at 77 K by a II-VI group compound semiconductor laser of ZnCdSe/ZnSe single quantum-well (SQW) structure, which oscillates in the visible short-wavelength region. This report said that the reflector for an optical resonator of the laser used the cleavage plane originating from the crystal cleavage as it was. The reflectance at the end-faces was about 20%, the threshold current in the oscillation was 74 mA, and the threshold current density was 320 A/cm.sup.2.
Further, Jpn. J. Appl. Phys. Vol. 29 (1990) p.L205 disclosed that a III-V group compound semiconductor, GaN, produced a stimulated emission of a 375 nm near-ultraviolet light by light emission at room temperature. The threshold excitation density was 700 kW/cm.sup.2 and the equivalent current density reached 210 kA/cm.sup.2. The reflector for an optical resonator also used a cleavage plane as it was. The reflectance at the end-face was 25% or less.
Moreover, an experiment on a near ultraviolet light emitting diode of a pn Junction type GaN was reported by the 11th Symposium Record of Alloy Semiconductor Physics and Electronics (1992) p.127.
On the other hand, for conventional III-V group compound semiconductor red lasers, which show an oscillating wavelength of 670 to 690 nm, the reflectance at the end-faces has been optimized by forming an alternately deposited multilayer film of, for example, amorphous Si and SiO.sub.2 on a cleavage plane of a device. The formation of the film was done with sputtering method, vacuum deposition, chemical vapor deposition or the like.
Direct use of the cleavage plane of a device as a reflector increases the threshold current due to low reflectance of the reflector, and it cannot effectively extract laser output. Consequently, to optimize the reflectance, a reflector of semiconductors, metals or dielectrics must be formed on the end-faces.
It is, however, difficult to apply the kind of reflector used in the III-V group compound semiconductor laser to a short-wavelength semiconductor laser because these semiconductor lasers differ from each other in the oscillating wavelength region. A reflector of, for example, amorphous Si, increases the imaginary component of a complex refractive index at 500 nm to near the real number of the index; this reflector absorbs emitted light and increases the threshold current. It causes optical damage by the laser light to the end-faces of the device. Although a reflector of a metal such as gold (Au) is a good reflector having no absorption in the infrared region, we cannot use it because it exhibits a large absorption in the region of wavelength shorter than the visible wavelength. Furthermore, laser behavior depends upon factors such as (i) the difference in thermal expansion coefficients between a reflector material and the device, (ii) adhesion of the reflector to the device, (iii) crystallization of the reflector, (iv) hardness of the reflector, (v) wet-proofness of the reflector, (vi) stability of the reflector, and the like. These factors cause in particular strain in an active layer of the device, and optical damage to the device. They are also practically important. Therefore, we need to optimize the factors.
The invention aims to provide a reflector essential to the best behavior of a semiconductor device which oscillates in the region from the near infrared region to the visible and short wavelength region, and a method of manufacturing the reflector.